This invention relates to a process for producing a foam sheet having an embossed pattern and, more particularly, to a process for producing a foam sheet having an embossed pattern by applying a coating containing an aliphatic ketone type solvent as a main ingredient over a foamable polyvinyl chloride resin sheet material containing a specific amine or amines and a blowing agent comprising azodicarbonamide as a main ingredient and subsequently heating the foamable sheet.
Known in the art of producing foam sheets having embossed patterns are processes as disclosed in the specifications of Japanese Patent Publication No. 15713/1968, U.S. Pat. No. 3,293,108, U.S. Pat. No. 3,293,094 (Japanese Pat. No. 578566) and Japanese Patent Publications Nos. 4913/1971 and 13929/1971 according to which embossed patterns are formed on foam sheets by applying a coating added with a compound which either inhibits or accelerates decomposition of a blowing agent over the surface of foamable synthetic resin sheets containing the blowing agent which is decomposed by heating and subsequently heating the foamable sheets to cause a differentiation in the degree of foaming in portions to which the coating has been applied from that in portions to which the coating has not been applied.
In these prior art processes, it has been necessary, as indicated above, to add a compound (e.g. an acid anhydride or amine) which inhibits or accelerates decomposition of the blowing agent to the coating. It has been found, however, that the addition of such compound (inhibitor or accelerator) to a common type of coating causes difficulty in using the coating in some cases due to occurrence of blushing or gelling or, even if such troubles do not occur, viscosity of the coating increases to such a degree that they can no longer be suitably used for printing purposes. For these reasons, the prior art processes are disadvantageous in that types of coatings to which such compounds having a function of inhibiting or accelerating decomposition of the blowing agent can be added without adversely affecting properties of the coatings are extremely limited.
It is, therefore, an object of the present invention to provide a novel process for producing a foam sheet which has eliminated the above described disadvantages of the prior art methods and is capable of forming a foam sheet having a distinct embossed pattern by using a coating of a common or conventional composition without addition of any foaming accelerator or inhibitor and, accordingly, is free from the likelihood of occurrence of blushing, gelling or viscosity increase in the coatings.
Laborious studies may be the inventors of the present invention on the mechanism of foaming inhibition in the prior art processes have resulted in the finding, which has led to the present invention, that while specific amines to be described later do not function to reduce the quantity of a gas which is given off as a result of decomposition of a blowing agent containing azodicarbonamide as a main ingredient if these amines alone are used with such blowing agent, these amines function to reduce the quantity of such gas to a great extent if they are used together with aliphatic ketone type solvents among known solvents for coating which likewise have no function of reducing the quantity of the gas given off as a result of the decomposition of the blowing agent containing azodicarbonamide as its main ingredient if these solvents alone are used with the blowing agent. Consequently, according to the finding, application of a common coating which contains the aliphatic ketone type solvent as a main ingredient and no added compound functioning to inhibit or accelerate decomposition of the blowing agent comprising azodicarbonamide as its main ingredient to a foamable synthetic resin sheet containing the blowing agent can remarkably inhibit foaming of the foamable sheet in portions to which the coating has been applied, if the above described specific amine or amines have been added to the formable sheet, and a foam sheet having a desired embossed pattern can thereby be produced.